Drain Radiant Floor Loops

I have a slab foundation on my workshop and we have decided to winterize it for the winter due to the high costs of fuel to heat it. With that said...

My workshop floor has 6 1/2" pex loops in it and a on demand water heater to heat the water. I want to drain out the pex loops but cannot figure out how to get the air pumping into the loops accordingly. Can anyone point me in the right direction?